Two explanations for the accelerating expansion of the universe

The surprise isn't that the universe expands. Cosmologists have known that since 1929. The surprise is that its expansion is speeding up. They had expected gravity to tug against the expansion and gradually slow it down, but the distant supernovas were farther away than that expectation allowed.

Separate the observation from its explanation. The distances provide evidence of acceleration. They don't yet tell us what causes it. The rest of the discussion introduces two ways to account for the same finding.

The first explanation proposes dark energy, which causes space itself to stretch. We don't need to supply a physics lesson that the text hasn't given us. For this argument, the useful idea is that something associated with space could drive accelerated expansion. Studies of the big bang's afterglow support the proposal, and the author says it is the more widely accepted theory. That is support, not a declaration that the question is settled.

The simplest version treats dark energy as an inherent part of the vacuum, exerting constant pressure. Einstein had proposed a similar quantity, the cosmological constant, in 1917. His original problem was different: why wasn't the universe shrinking under its own gravity? Once expansion was discovered, he abandoned the idea as unnecessary. Now an old idea might help answer a new question—why the expansion accelerates. Similar concepts can be used for different explanatory purposes.

The third paragraph asks where that energy might come from. The so-called vacuum isn't simply empty: quantum mechanics describes virtual subatomic particles appearing and disappearing, producing constant vacuum energy. Some cosmologists propose that this is the dark energy needed by their explanation.

Then comes a problem with that proposal. The predicted vacuum energy doesn't match the amount needed to explain the acceleration. Giving the energy a possible source isn't enough if the quantities don't line up. We aren't told here whether the prediction is too high or too low; the significant point is the large discrepancy between the prediction and the requirement.

The second explanation avoids that particular problem because it doesn't rely on dark energy. It proposes that gravity behaves differently across enormous distances from what Einstein's general theory of relativity predicts. Instead of adding an energy source, it changes the account of how gravity works.

That would be a radical change. But the author also identifies a practical advantage: a revised account of gravity on very large scales would presumably affect predictions on smaller scales. We might test those consequences within our own solar system. The connection matters. A theory about distances billions of light-years across needn't be assessed only by observing those remote distances if it also has consequences nearby.

Easier to test is not the same as already confirmed. The final sentence offers a possible advantage in evaluating the second theory, not a verdict that it is correct or that dark energy cannot be tested. We finish with two explanations, each challenging current science: one has a problem matching predicted energy to the observed acceleration; the other requires a substantial revision of gravity. Our main point should retain both proposals and their difficulties.